In general, dot matrix printers can be separated into two categories--dot matrix line printers and dot matrix serial printers. Both categories of printers create images (characters or designs) by selectively printing a series of dots in an X-Y matrix. Dot matrix serial printers include a print head that is moved horizontally back and forth across a sheet of paper, either continuously or in steps. The print head includes a vertical column of dot printing elements. As each column position of a character position is reached during printing, the required number of dot-printing elements are actuated to form dots. A series of thusly created vertical dot columns form the desired character. Contrariwise, dot matrix line printers include a dot-printing mechanism for creating horizontal lines and dots substantially simultaneously as paper is stepped through the printer. A series of horizontal lines of dots creates an image, i.e., a row of characters or a design. While the present invention may find use in other areas, because it was designed for use in dot matrix printers, it is described in connection with such a printer.
In the past, various types of actuating mechanisms for use in dot matrix printers have been proposed and implemented. Examples of actuating mechanisms for use in dot matrix line printers are described in U.S. Pat. No. 4,503,768, entitled "Single Piece Hammer Module" and U.S. Pat. No. 4,584,937, entitled "Long Release Coil Hammer Actuating Mechanism," both assigned to the assignee of the present application. Both of these patents describe actuating mechanisms incorporated in hammer modules that include a plurality of cantilever mounted print hammer arms formed of a resilient ferromagnetic material. Located on the end of each print hammer arm is an anvil (e.g., a ball) that prints a dot when the associated hammer arm is actuated. Each hammer actuation mechanism comprises a permanent magnet, a post and plates that create a magnetic path between the permanent magnet and the post, plus a release coil mounted on the post. In the absence of current through the release coil, the print hammer arm is attracted to the post by the magnetic field produced by the permanent magnet. The attraction stresses the hammer arm. The thusly cocked hammer arm is released by energizing a release coil such that the coil produces a magnetic field that counteracts the magnetic post attraction field created by the permanent magnet. When released, the stored energy resulting from stressing a resilient hammer arm causes the hammer to impact the anvil against the ribbon and create a dot on a print receiving medium.
While dot-printing mechanisms of the type generally described above have a number of advantages over previously developed dot-printing mechanisms for use in dot matrix line printers and, thus, form a significant step forward in this art, it has been found that such dot printing mechanisms can be improved. In this regard, in the past, the posts of dot matrix serial and line printer actuators of the type described above have been formed of wear resistant, magnetically permeable materials. While, from a wear point of view, such materials having long lifetimes, from a magnetic point of view they are undesirable because of their low resistivity. As a result of their low resistivity, when current is applied to the release coil to release a hammer to create a dot, the magnetization of the coil post changes. The magnetization change creates eddy currents, resulting in a loss of power. The eddy current power loss generates undesirable heat. In addition, since eddy currents produce an opposing magnetic field they slow down hammer release. In order to compensate for this slowing effect, coil drive current must be increased if the desired speed is to be maintained.
Other types of magnetic devices, such as transformers and motors have dramatically reduced eddy current power losses by the use of laminated cores formed of thin, flat sheets of a high resistivity magnetic material. Frequently, the chosen material also has low hysteresis loss. Unfortunately, high resistivity materials wear when subjected to mechanical forces. Thus, in the past, the need for wear resistance at the point where the hammer impacts the post, i.e., the post tip, has prevented the use of the magnetically most desirable post materials. This invention is designed to overcome this disadvantage by providing a coil post for a print actuator, in particular a dot matrix line or serial print actuator, that minimizes eddy currents and, thus, the power loss resulting from eddy currents, while satisfying the need for wear resistance at the point where the hammer impacts the post.